bundled of BioShock and Bioshock 2 for PC for $10.
A few years ago, Bioshock had some of the best reviews of any 3D first-person-shooter game of all-time.
Video review.
If you enjoy this kind of action/adventure 3D shooter game and love a good story, and have not yet played BioShock, we highly recommend it.
Intended for mature audiences.